The
reaction pathway of the oxygen reduction reaction (ORR) is
strongly affected by the electrolytic environment. Meanwhile, the
ORR mechanism on transition-metal oxide catalysts has not been studied
intensely in very concentrated alkaline solutions that are used in
practical metal–air batteries. Herein, we report the in situ activation of ORR catalysis on manganese perovskite
in a concentrated alkaline solution, mediated by the spontaneous formation
of oxygen vacancy sites. Electrochemical analyses of the (100) epitaxial
film electrodes reveal that the exchange current and electron number
of the ORR on La0.7Sr0.3Mn0.9Ni0.1O3 significantly increase with the duration of
the ORR when the KOH concentration is greater than 4 M. However, these
values remain unchanged with time at less than 1 M KOH concentration.
Operando synchrotron X-ray spectroscopy of the (100) epitaxial film
confirmed that La0.7Sr0.3Mn0.9Ni0.1O3 involves the oxygen vacancy sites with the
reduction of Mn atoms in concentrated KOH solution via the hydroxylation
decomposition of perhydroxyl intermediates. Hence, the O2 adsorption switched from an end-on to a bidentate mode because the
cooperative active sites of the oxygen vacancy and neighboring Mn
allow bidentate adsorption of the dissolved O2. Due to
the simultaneous interaction with the oxygen vacancy and Mn sites,
the O–O bonds are activated and the potential barrier for the
electron transfer to adsorbed O2 is lowered, resulting
in a shift in the reaction mechanism from that involving an indirect
“2 + 2” transfer pathway to a direct 4-electron pathway.
Brownmillerite-type Ca2FeCoO5 (CFCO) is a highly active electrocatalyst for the oxygen evolution reaction (OER). In this study, we identified the actual catalytically active phase of this oxide formed via the long-term OER and, moreover, demonstrated that the active phase can persist during the OER for four weeks without significant loss of electrocatalytic activity. The long-term durability tests were carried out on CFCO via continuous galvanostatic OER in 4 mol dm -3 KOH aqueous solution for periods ranging from a few hours to one month, and the specimens submitted to the tests were characterized by means of electrochemical measurements and structural analysis using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, Auger electron spectroscopy, and X-ray absorption fluorescence spectroscopy. CFCO was readily converted to amorphous cobalt oxyhydroxides with 10% of Fe substituents through the OER process, and these compounds had a similar local rearrangement to the layered γ-CoOOH-type structure. This transformation involved large morphological changes of the oxide particles because of the extensive dissolution of Ca and Fe, yielding skeletal grains made of oxyhydroxide nanosheet aggregates. The extended durability studies with total polarization charge density of the order of 10 5 C cm -2 revealed that (Co, Fe)OOH like compound is the actual electrocatalytic phase.
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